Age-specific Risk of SARS-CoV-2 Reinfection During Omicron Outbreaks, South Korea.

We assessed the risk of reinfection among all residents in South Korea who tested positive for coronavirus disease 2019 from January to August 2022. Children 5-11 years [adjusted hazard ratio (aHR) = 2.20], and 12-17 years old (aHR = 2.00), were at higher risk; whereas 3-dose vaccination (aHR = 0.20) lowered the risk of reinfection.

Effectiveness of Heterologous COVID-19 Vaccine Booster in Korean Elderly Population, 2022.

We conducted a cohort study to assess vaccine effectiveness (VE) of coronavirus disease 2019 vaccine combinations on severe acute respiratory syndrome coronavirus 2 critical infection and death among elderly population in Korea. From January to August 2022, VE against death for 4 doses mRNA recipients was 96.1%, whereas 1-dose viral vector + 3-dose mRNA recipients had VE of 90.8%.

Analysis of pregnant women with critically severe COVID-19 in Republic of Korea from February 2020 and December 2021.

OBJECTIVES: This study aimed to describe the characteristics and risk factors for severe disease in pregnant women infected with coronavirus disease 2019 (COVID-19) from the early days of the COVID-19 epidemic in Korea to the predominant period of the Delta variant. METHODS: A retrospective cohort study was conducted among pregnant women diagnosed with COVID-19 between February 2020 and December 2021. Logistic regression analysis was performed to compare severe and mild cases after adjusting for pregnant women's age, nationality, infection route, outbreak area, infection period, symptoms, underlying disease, smoking status, trimester, and COVID-19 vaccination status. RESULTS: In total, 2,233 pregnant women were diagnosed with COVID-19 by December 2021. Among these, 96.7% had mild symptoms, 3.3% had severe symptoms, and 0.04% died. The risk factors for severe disease in pregnant women with confirmed COVID-19 were being in the age group of 35 to 45 years, having hyperlipidemia, being in the second or third trimester of pregnancy at the time of COVID-19 diagnosis, being infected during the Delta-predominant period, and having a fever (≥38 °C) at diagnosis. Furthermore, 47.1% of patients in the mild group and 84.9% of patients in the severe group had 3 or more risk factors. CONCLUSION: Pregnant women with COVID-19 mainly experienced mild symptoms, but those with risk factors were at a higher risk of developing severe symptoms. Therefore, treatment and follow-up management should be thoroughly implemented.

COVID-19 Outbreak in a Military Unit in Korea.

OBJECTIVES: This study presents the response of a military unit to the COVID-19 outbreak in Gyeonggi Province. As soon as two soldiers were identified as index cases, the infectious disease investigators of the Gyeonggi Provincial Government, Korea Disease Control and Prevention Agency and the Armed Forces Epidemiologic Investigation Center, discussed the investigation and response plan for an imminent massive outbreak. METHODS: The joint immediate response team (IRT) conducted interviews with confirmed patients with COVID-19, reviewed medical records, performed contact tracing using global positioning system (GPS), and undertook a field investigation. For risk assessment, the joint IRT visited all eight sites of the military units and the army chaplain's church to evaluate the transmission risk of each site. The evaluation items included the size of the site, the use of air conditioning, whether windows were opened, and whether masks were worn. A pooled testing was used for a low-risk population to quickly detect the spread of COVID-19 in the military base. RESULTS: A day before the symptom onset of the index case, the lecturer and >50% of the attendees were infected with COVID-19 while attending a lecture that lasted 2 h and 30 min. Attendees were not wearing masks and were in a poorly ventilated room. CONCLUSION: Since the disease can be spread before symptom onset, contact tracing must be performed to investigate potential exposures prior to symptom onset and manage any exposed persons.

Vaccine Effectiveness Against Severe Disease and Death for Patients With COVID-19 During the Delta-Dominant and Omicron-Emerging Periods: A K-COVE Study.

National cohort data collected during the coronavirus disease 2019 (COVID-19) delta and omicron periods in Korea revealed a lower risk of severe infection in recipients of three doses of the COVID-19 vaccine (adjusted odds ratio [aOR], 0.05-0.08). The risk of death was reduced during the omicron period compared to the delta period (aOR, 0.75; 95% confidence interval, 0.67-0.84).

A Seroprevalence Study on Residents in a Senior Care Facility with Breakthrough SARS-CoV-2 Omicron Infection.

The Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) began spreading rapidly in the community in November 2021, becoming the dominant variant in the Republic of Korea in 2022. Although its pathogenesis in healthy individuals was low, the severity and hospitalization rate was higher in the elderly and immunocompromised patients. We aimed to investigate the immunogenicity in acute and convalescent phases of breakthrough infection by Omicron in elderly individuals. Serological data were assessed by electrochemiluminescence immunoassay, enzyme-linked immunosorbent assay, and plaque-reduction neutralization tests. SARS-CoV-2-specific antibody and immunoglobulin G levels in the acute phase were higher in third dose-vaccinated elderly than in first and second dose-vaccinated patients. The neutralization antibody titer was detected only in third dose-vaccinated patients, and the titer was higher for the Delta than the Omicron variant. In the convalescent phase of Omicron infection, the neutralization antibody titer of vaccinated patients was higher for the Delta than the Omicron variant except in unvaccinated individuals. We demonstrated that the cause of the vulnerability to Omicron variant infection in third dose-vaccinated elderly was due to the low neutralization antibody level against Omicron. A fourth dose of vaccination is required in the elderly to reduce hospitalization and mortality caused by the Omicron variant.

Early countermeasures to COVID-19 at long-term care facilities in Gwangju Metropolitan City, Republic of Korea.

OBJECTIVES: The coronavirus disease 2019 (COVID-19) pandemic has continued since its first detection in the Republic of Korea on January 20, 2020. This study describes the early countermeasures used to minimize the risk of COVID-19 outbreaks during cohort quarantine and compares the epidemiological characteristics of 2 outbreaks in long-term care facilities (LTCFs) in Gwangju Metropolitan City in summer 2020. METHODS: An epidemiological investigation was conducted via direct visits. We investigated epidemiological characteristics, including incidence, morbidity, and mortality rates, for all residents and staff members. Demographic characteristics were analyzed using a statistical program. Additionally, the method of managing infection in LTCFs is described. RESULTS: Residents and caregivers had high incidence rates in LTCF-A and LTCF-B, respectively. LTCF-B had a longer quarantine period than LTCF-A. The attack rate was 20.02% in LTCF-A and 27.9% in LTCF-B. The mortality rate was 2.3% (1/43) in LTCF-B, the only facility in which a COVID-19 death occurred. CONCLUSIONS: Extensive management requires contact minimization, which involves testing all contacts to mitigate further transmission in the early stages of LTCF outbreaks. The findings of this study can help inform and prepare public health authorities for COVID-19 outbreaks, particularly for early control in vulnerable facilities.

Effectiveness of A Fourth Dose of COVID-19 mRNA Vaccine in the Elderly Population During the Omicron BA.2 and BA.5 Circulation: A Nationwide Cohort Study in Korea (K-COVE).

We estimate the effectiveness of a fourth dose booster of coronavirus disease 2019 mRNA vaccine in individuals aged ≥60 years during Omicron BA.2 and BA.5 circulation in Korea. The effectiveness against critical infection was 67.7% (95% confidence interval, 50.7%-78.8%) at 31-60 days and 62.1% (95% confidence interval, 45.5%-73.7%) at 61-90 days.

Estimated Effectiveness of Prior SARS-CoV-2 BA.1 or BA.2 Infection and Booster Vaccination Against Omicron BA.5 Subvariant Infection.

This case-control study estimates the effectiveness of prior SARS-CoV-2 BA.1 or BA.2 infection and booster vaccination against Omicron BA.5 subvariant infection.

Positive Predictive Value of Rapid Antigen Tests in School During SARS-CoV-2 Omicron Variant Surge.

In Korea, we conducted a national observational study to calculate the positive predictive value of SARS-CoV-2 rapid antigen tests in K-12 schools during the Omicron variant surge in March 2022. The weekly positive predictive value ranged from 86.4% to 93.2%. The positive predictive value was the highest among elementary school students with symptoms (95.7%) and lowest among teachers/staff without symptoms (70.9%).

BNT162b2 Vaccine Effectiveness Against the SARS-CoV-2 Omicron Variant in Children Aged 5 to 11 Years.

This cohort study examines data for all children aged 5 to 11 years in South Korea to gauge the effectiveness of the BNT162b2 vaccine when the Omicron variant was the dominant SARS-CoV-2 infection in the country.

The effectiveness of Paxlovid treatment in long-term care facilities in South Korea during the outbreak of the Omicron variant of SARS-CoV-2.

OBJECTIVES: On November 5, 2021, Pfizer Inc. announced Paxlovid (nirmatrelvir +ritonavir) asa treatment method that could reduce the risk of hospitalization or death for patients withconfirmed coronavirus disease 2019 (COVID-19). METHODS: From February 6, 2022 to April 2, 2022, the incidence of COVID-19 and the effectsof treatment with Paxlovid were analyzed in 2,241 patients and workers at 5 long-term carefacilities during the outbreak of the Omicron variant of severe acute respiratory syndromecoronavirus 2 in South Korea. RESULTS: The rate of severe illness or death in the group given Paxlovid was 51% lower thanthat of the non-Paxlovid group (adjusted risk ratio [aRR], 0.49; 95% confidence interval [CI],0.24-0.98). Compared to unvaccinated patients, patients who had completed 3 doses of thevaccine had a 71% reduced rate of severe illness or death (aRR, 0.29; 95% CI, 0.13-0.64) and a65% reduced death rate (aRR, 0.35; 95% CI, 0.15-0.79). CONCLUSION: Patients given Paxlovid showed a lower rate of severe illness or death and alower fatality rate than those who did not receive Paxlovid. Patients who received 3 dosesof the vaccine had a lower rate of severe illness or death and a lower fatality rate than theunvaccinated group.

mRNA vaccine effectiveness against SARS-CoV-2 B.1.617.2 (Delta) and B.1.1.529 (Omicron) variant transmission from home care cases to household contacts in South Korea.

OBJECTIVES: Household contacts of confirmed cases of coronavirus disease 2019 (COVID-19) areexposed to a high risk of viral transmission, and secondary incidence is an important indicatorof community transmission. This study analyzed the secondary attack rate and mRNA vaccineeffectiveness against transmission (VET) for index cases (patients treated at home) confirmedto be infected with the Delta and Omicron variants. METHODS: The subjects of the study were 4,450 index cases and 10,382 household contacts.Logistic regression analysis was performed to compare the secondary attack rate byvaccination status, and adjusted relative risk and 95% confidence intervals were identified. RESULTS: The secondary attack rate of the Delta variant was 27.3%, while the secondary attackrate of the Omicron variant was 29.8%. For the Delta variant, groups with less than 90 daysand more than 90 days after 2 doses of mRNA vaccination both showed a VET of 37%. For theOmicron variant, a 64% VET was found among those with less than 90 days after 2 doses ofmRNA vaccination. CONCLUSION: This study provides useful data on the secondary attack rate and VET of mRNAvaccines for household contacts of COVID-19 cases in South Korea.

Spatial distribution of SARS-CoV-2 infection in schools, South Korea.

Identification of geographical areas with high burden of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission in schools using spatial analyses has become an important tool to guide targeted interventions in educational setting. In this study, we aimed to explore the spatial distribution and determinants of coronavirus disease 2019 (COVID-19) among students aged 3-18 years in South Korea. We analysed the nationwide epidemiological data on laboratory-confirmed COVID-19 cases in schools and in the communities between January 2020 and October 2021 in South Korea. To explore the spatial distribution, the global Moran's I and Getis-Ord's G using incidence rates among the districts of aged 3-18 years and 30-59 years. Spatial regression analysis was performed to find sociodemographic predictors of the COVID-19 attack rate in schools and in the communities. The global spatial correlation estimated by Moran's I was 0.647 for the community population and 0.350 for the student population, suggesting that the students were spatially less correlated than the community-level outbreak of SARS-CoV-2. In schools, attack rate of adults aged 30-59 years in the community was associated with increased risk of transmission (P < 0.0001). Number of students per class (in kindergartens, primary schools, middle schools and high schools) did not show significant association with the school transmission of SARS-CoV-2. In South Korea, COVID-19 in students had spatial variations across the country. Statistically significant high hotspots of SARS-CoV-2 transmission among students were found in the capital area, with dense population level and high COVID-19 burden among adults aged 30-59 years. Our finding suggests that controlling community-level burden of COVID-19 can help in preventing SARS-CoV-2 infection in school-aged children.

Presumed population immunity to SARS-CoV-2 in South Korea, April 2022.

OBJECTIVES: We estimated the overall and age-specific percentages of the Korean population with presumed immunity against severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) as of April 2022 using the national registry. METHODS: We used the national coronavirus disease 2019 (COVID-19) infection and vaccination registry from South Korea, as described to define individuals with a previous history of COVID-19 infection, vaccination, or both, as persons with presumed immunity. RESULTS: Of a total of 53,304,627 observed persons, 24.4% had vaccination and infection, 58.1% had vaccination and no infection, 7.6% had infection and no vaccination, and 9.9% had no immunity. The SARS-CoV-2 Omicron variant emerged at a time when the presumed population immunity ranged from 80% to 85%; however, nearly half of the children were presumed to have no immunity. CONCLUSION: We report a gap in population immunity, with lower presumed protection in children than in adults. The approach presented in this work can provide valuable informed tools to assist vaccine policy-making at a national level.

Effectiveness of Second mRNA COVID-19 Booster Vaccine in Immunocompromised Persons and Long-Term Care Facility Residents.

We used a nationwide population registry in South Korea to estimate the effect of a second booster dose of mRNA COVID-19 vaccine on the risk for laboratory-confirmed SARS-CoV-2 infection, critical infection, and death in immunocompromised persons and long-term care facility (LTCF) residents. During February 16-May 7, 2022, among 972,449 eligible persons, 736,439 (75.7%) received a first booster and 236,010 (24.3%) persons received a second booster. Compared with the first booster group, at 30-53 days, the second booster recipients had vaccine effectiveness (VE) against all infections of 22.28% (95% CI 19.35%-25.11%), VE against critical infection of 56.95% (95% CI 29.99%-73.53%), and VE against death of 62.96% (95% CI 34.18%-79.15%). Our findings provide real-world evidence that a second booster dose of mRNA vaccine substantially increases protection against critical infection and death in these high-risk population groups.

Characteristics of COVID-19 outbreaks and risk factors for transmission at an army training center in South Korea from June to August 2021.

OBJECTIVES: It is crucial to establish the characteristics of coronavirus disease 2019 (COVID-19) outbreaks at army training centers to develop preventive measures. Therefore, this study aimed to determine the COVID-19 transmission patterns and risk factors in a sequence of outbreaks at an army training center from June to August 2021. METHODS: This study included 1,324 trainees at an army training center where outbreaks occurred from June to August 2021. The outbreak was qualitatively analyzed according to the period, attack rate, demographic characteristics, vaccination history, and living areas. An aerodynamic experiment was performed to evaluate aerosol transmission in living areas. RESULTS: Three outbreaks occurred at the army training center from June to August 2021. The first, second, and third outbreaks lasted for 32, 17, and 24 days, and the attack rates were 12.8%, 18.1%, and 8.9%, respectively. Confirmed cases were distributed in all age groups. Recruits and the unvaccinated were at higher risk for COVID-19. The aerodynamic experiment verified the possibility of aerosol transmission within the same living area. CONCLUSION: COVID-19 transmission at army training centers should be minimized through quarantine and post-admission testing during the latency period as part of integrated measures that include facility ventilation, vaccination, indoor mask-wearing, and social distancing.

Analysis of the on-ship transmission of the COVID-19 mass outbreak on the Republic of Korea Navy amphibious warfare ship.

OBJECTIVES: This investigation was conducted to determine the size and pattern, source, and transmission route of the coronavirus disease 2019 (COVID-19) outbreak on the Republic of Korea Navy (ROKN) amphibious warfare ship. METHODS: We investigated the characteristics of all crew members and tracked the medical records of the confirmed cases. Fourteen essential ship operation personnel were interviewed. The study design was a retrospective cohort study, and the incidence rate ratio was through a statistical program. RESULTS: The COVID-19 incidence on the ROKN amphibious warfare ship was 44.7% (38/85). It was estimated that the main propagation route started from the 1st floor worker, which spread to the same floor, and then to other floors. In the case of the working area, the incidence rate of crew members below the 1st floor without ventilation was higher than those on the 2nd or higher floors with natural ventilation. CONCLUSIONS: This case is the first case of a COVID-19 outbreak on the ROKN amphibious warfare ship, and it is estimated that the incidence rate is high because of the closed and dense environment. To prevent the spread of various respiratory diseases including COVID-19, unified mitigation such as vaccination, observing personal quarantine rules, periodic ventilation, preemptive testing, and blocking transmission through prompt contact management is necessary.